The present invention relates to power tongs typically used in the oil and gas industry to make up and break apart threaded joints on pipe, casing and similar tubular members. In particular, this invention relates to an improved gear train used within power tongs.
Power tongs have been in existence for many years and are generally employed in the oil and gas industry to grip and rotate tubular members, such as drill pipe. It is necessary to grip drill pipe with high compressive forces while applying a high degree of torque in order to break apart or tighten threaded pipe connections. In most cases, power tong designs employ a cam mechanism for converting a portion of the torque into a gripping (compressive) force normal to the pipe. This conversion is often accomplished utilizing a power-driven ring gear having an interior cam surface. A cam follower (roller) on ajaw member rides upon the cam surface. As the ring gear is rotated, the follower (and thus the jaw member) is urged into contact with the pipe.
For purposes of describing the prior art, reference may be made to the power tong seen in FIG. 1. Power tong 1 has a body 2 a top plate 4 which is shown partially cut away in FIG. 1. A, ring gear 6 is rotatively mounted within body 2 on roller bearings 8 and includes a set of neutral cam surfaces 17 and positive cam surfaces 18 formed on the inner perimeter of ring gear 6. Cage plates 16 are positioned within ring gear 6 (although the top cage plate has been removed for clarity in FIG. 1) and act as jaw carriers for jaw members 15. Power tong body 2 has an open throat 5 and ring gear 6 as a corresponding open throat 10. While throat 10 is shown out of alignment with throat 5 in FIG. 1, it will be understood that throat 5 and throat 10 may be aligned in a "neutral" position to allow the insertion of a tubular member 14 into tong body 2. Power tong 1 grips tubular 14 by way of relative movement between cage plate 16 and ring gear 6. When ring gear 6 is rotated out of the neutral position as shown in FIG. 1, the jaw members 15 move onto positive cam surfaces 18 and grip tubular 14. Once tubular 14 is griped, ring gear 6 will continue rotating in order to connect or disconnect a threaded joint formed between two tubulars. The foregoing is well known in the art and disclosed in references such as U.S. Pat. No. 5,291,808 to Buck and U.S. Pat. No. 4,084,453 to Eckel, both of which are incorporated by reference herein.
The rotation of ring gear 6 is caused by the power tong gear train 19 which is shown schematically in FIG. 1 and can be seen more clearly in FIG. 2. FIG. 2 illustrates the mechanical relationship of ring gear 6 to gear train 19, but omits details of ring gear 6 (such as cam surfaces) which are not necessary to the understanding of the present invention. Ring gear 6 will have a series of teeth 7 around its perimeter except for the opening of throat 10. Gear train 19 comprises the set of gears transferring power from motor 24 to ring gear 6. All gears in gear train 19 have teeth 7 and are mounted on a gear bearing shaft 23 upon which the gears may freely rotate, all of which is well known in the art. In the illustrated gear train 19, a set of idler gears 25 engage and transfer torque to ring gear 6. Another gear 22 transfers torque to idler gears 25 and gear 22 in turn has torque transferred to it by pinion gear 26. Pinion gear 26 comprises an upper pinion gear 27 and a lower pinion gear 28. It will be understood that upper pinion gear 27 and a lower pinion gear 28 are fixed to one another and must rotate in unison as is well known in the art. Lower pinion gear 28 engages motor gear 29 such that torque may be transferred from motor 24, through gear train 19, to ring gear 6. It will be readily apparent that one purpose of gear train 19 is to convert the relatively high speed, low torque rotation of motor 24 to lower speed, higher torque rotation at ring gear 6. Thus, the gear train will have at least one and typically several stages of speed reduction and torque elevation. Each reduction stage is accomplished by transferring power from a gear with a smaller number to teeth to a gear with a larger number of teeth. In FIG. 2, it can be seen that there is a first reduction stage between motor gear 29 and lower pinion gear 28, a second reduction stage between upper pinion gear 27 and gear 22, and a third reduction stage between gear 22 and ring gear 6.
While the gear train 19 of FIG. 2 has been the standard in the power tong industry, it suffers from certain serious disadvantages. First, it can be seen that with every revolution of ring gear 6, there is a moment of time when throat 10 encounters each of the idler gears 25 and causes that idler gear 25 to cease transferring torque to ring gear 6. Therefore, the idler gear 25 not encountering throat 10 must bear the entire torque load being transferred to ring gear 6. This torque load can be considerable when the power tong is torquing up a fully threaded connection or is breaking apart a tightly connected joint. This large and unbalanced torque load acting on only one idler gear 25 introduces undesirable stresses into gear train 19. Furthermore, since each idler gear must be designed to bear the entire torque load, the gears must be sized accordingly. This results in the gears having an increased thickness or depth. For clarity, this depth dimension "d" is shown in FIG. 4. The increased size of individual gears in gear train 19 is undesirable on account of the increase in weight and material costs. Moreover, the weight and cost disadvantages are considerably increased in relation to ring gear 6, whose depth dimension must match that of idler gears 25. As is apparent from FIG. 2, ring gear 6 is by far the most substantial gear component of the power tong. Thus, increasing the size of ring gear 6 substantially increases the overall weight and materials cost of the power tong.
Another disadvantage of the prior art gear train is unequal loading across the idler gears caused by the initial application of torque to a stationary ring gear 6. Power tongs typically operate in an environment where drilling mud, grit and small solids are likely to come into contact with the gear train. Gear trains are consequently designed so that the teeth 7 mesh with comparatively loose tolerances so that grit and outside contaminants are less likely to obstruct the operation of the gear train. However, this loose tolerance design means that when idler gears 25 begin turning to apply torque to a stationary ring gear 6, the teeth 7 of both idler gears 25 do not necessarily engage teeth 7 of the ring gear 6 at precisely the same moment. For example, the teeth 7 of the lower idler gear 25 seen in FIG. 2 may be in actual contact with teeth 7 of ring gear 6, while the teeth 7 of the upper idler gear 25 may be just short of contact with the teeth 7 of ring gear 6. Thus, when motor 24 begins transferring power through gear train 19, lower idler gear 25 will for a short time bear a much greater load than upper idler gear 25. Any such momentary unbalanced load situation creates damaging stresses throughout the gear train. These stresses particularly effect the shafts and bearings upon which the gears rotate.
A third disadvantage with prior art gear trains relates to stresses generated when the ring gear abruptly stops rotating. For example, when working with tubulars such as API drill pipe which has free running threads, the female section of the threaded joint will have an internal shoulder which the male section will contact prior to the full length of the threads becoming engaged. Thus, the tubular will abruptly stop rotating as the male section contacts the shoulder, causing ring gear 6 to likewise abruptly stop. The stress of this abrupt stoppage is of course transferred through the entire gear train and is a highly undesirable effect.